The present invention relates to methods of mineless connection of wells for underground gasification and, more particularly, to methods of filtrational fire connection.
The herein disclosed method of connection can be successfully utilized for underground gasification of coal, when a unitary gasification channel has to be formed.
In addition, the present invention can be utilized for underground gasification of oil and kerosene shale and of oil-bearing beds.
There are known techniques of filtrational fire connection of wells, viz. the direct-flow technique and the counter-flow technique.
In the direct-flow technique, the coal bed is ignited in one of the wells intended for performing underground gasification of this coal bed, and a center of combustion is set. To connect the well in which the coal bed is ignited with the next successive well in the direction of formation of the unitary gasification channel, a blast of air is directed into the first-mentioned well. The blast may contain either ambient air or air enriched with oxygen. As a result, the zone of combustion starts expanding, owing to gassing out of the coal bed. Gassing out of the coal bed develops in accordance with the laws defining the paths of the blast of the gas through the body of the coal bed, depending on the direction and prevailing location of natural cracks. fissures and pores of the coal bed, with the outline of the gassed-out space elongating in the direction of these prevailing locations. More often than not this direction does not coincide with the required of formation of the gasification channel, and great quantities of coal are gassed out in order to effect connection of the two wells, which involves a waste of energy spent on maintaining the blast directed into the well.
Thus, a disadvantage of the direct-flow filtrational fire connection technique is the low intensity of formation of the gasification channel and the waste of the air blast. Another disadvantage of this technique is its poor directability, because gassing out of the coal bed takes place in the direction of the prevailing location of the fissures and pores of the coal bed.
On account of these disadvantages of the direct-flow filtrational fire connection technique, the technique of counter-flow filtrational fire connection has been recently utilized on an ever-increasing scale. In the counter-flow filtrational fire connection technique the coal bed is ignited in one of the wells to be connected, and a combustion center is set. The air blast is directed under pressure into the other well which is to be connected with the first-mentioned one. Owing to the presence of natural fissures and pores in the coal bed, the air blast directed into the well starts propagating through the coal bed in every direction according to the rules defined by the physical properties of the coal bed, such as its gas permeability, the presence of fissures, and the quantity of water in the coal bed. A portion of the blast propagating through the bed reaches the center of combustion, the oxygen contained in the blast reacts with the incandescent coal, and the heat thus produced heats the coal adjacent to the fire face on the side from which the blast has approached the combustion zone and ignites this coal. In this way the combustion zone advances in the direction of the propagation of the blast through the coal bed. Owing to the air blast being charged into the bed, the combustion zone advances through the coal bed until it reaches the well into which the blast is charged. The moment the combustion zone reaches the well into which the blast is charged, the pressure in this well drops considerably, usually, to 1 to 3 atm. gauge. This pressure drop indicates that the wells have been connected. When the wells have been connected, the coal gasification process is commenced.
To extend the initially formed gasification channel in the required direction, an air blast is charged into the next successive well in the required direction. When the latter has been connected with the previously formed channel, the well is operated for gasification, and the connecting blast is charged into the next successive well. In the required way a gasification channel is formed in the required direction.
It should be noted that the air blast for connecting a successive well with the already formed gasification channel is started after the preceding well has engaged in the gasification process. Since, as has been stated hereinabove, the operation of connecting the wells is accompanied by the pressure dropping in the wells to 1-3 atm. gauge, the inflow of underground water contained in the coal bed sharply increases at the moment of connection, which often results in the combustion center being extinguished. Besides, after the successive connected well has been engaged in the gasification process, the volume of the air blast charged thereinto is increased to provide for gasification of the coal bed. These two factors lead to extermination of the combustion center directly in the well with which connection has been effected because this combustion center is displaced into other areas of the gassed-out coal bed. This extermination, in its turn, impairs connection of the already formed gasification channel with the next successive well, since the combustion zone starts advancing not from the preceding well, but from more remote areas of the gassed-out coal bed. Thus, the process of connection of the wells becomes more time-consuming, and the volume of the blast which is to be charged to effect the connection is substantially increased. There are cases when the combustion center travels so far that connection of the wells becomes altogether impossible, and, therefore, it is impossible to form a unitary gasification channel. In such cases it becomes necessary to drill additional dewatering wells to dry the gasification areas.
Thus, among the disadvantages of the above described technique are its insufficient reliability in forming unitary gasification channel and its increased consumption of the air blast charged to effect connection.